1. Field of the Invention
This invention relates to a method for controlling a mixed refrigerant based, natural gas liquefier system through its startup, run and shutdown phases of operation. More particularly, this invention relates to a method for controlling a mixed refrigerant based, natural gas liquefier system that utilizes a novel exchange of system refrigerant between the system and an external storage tank whereby the use of extremely high pressures in the compressor discharge employed in conventional systems is circumvented, thereby allowing for the use of low cost, mass produced, HVAC components in the system.
2. Description of Related Art
Cryogenic liquefiers are commercially available for many industrial applications, including the liquefaction of natural gas to produce liquefied natural gas (LNG). These liquefiers are exclusively custom-made, permanent plants of large capacity commanding premium prices that the current market cannot afford. To date, the natural gas vehicle market has not generated enough incentives to develop a lowcost liquefier. However, the developing market and technology for natural gas vehicles provide a new opportunity for considering LNG fueling concepts requiring lower liquefaction installed facility costs. The recent growth in LNG demonstration programs and dedicated LNG-fueled fleets has created a market demand for a small shop-assembled liquefier in the 1000-3000 gallon/day capacity range. This low cost technology is being made possible through the identification and implementation of new refrigerant mixtures, refrigerant cycles and low cost HVAC mass produced components.
However, the control of a mixed refrigerant based natural gas liquefier using low cost HVAC components presents special problems, particularly in the system startup mode of operation in order not to exceed the pressure and temperature requirements of those components.
Accordingly, it is one object of this invention to provide a method of system control for a natural gas liquefaction system which enables operation of the system within the requirements of low cost components.
This and other objects of this invention are addressed by a method for controlling a natural gas liquefaction system comprising a refrigerant storage means, a refrigerant circulation means in fluid communication with the refrigerant storage means, and a natural gas liquefaction means in thermal communication with the refrigerant circulation means. The refrigerant storage means comprises a refrigerant holding tank and a refrigerant solenoid valve wherein the refrigerant solenoid valve controls the flow of refrigerant between the refrigerant holding tank and the refrigerant circulation means. The refrigerant circulation means comprises an oil flooded screw compressor having a suction pressure port, a discharge pressure port and an internal solenoid valve connecting the suction pressure port and the discharge pressure port; a variable speed motor suitable for driving the compressor operably connected to the compressor and an oil separator having a discharge gas inlet in fluid communication with the discharge pressure port, a separated oil outlet in fluid communication with an oil inlet of the compressor, and a separated gas outlet. A high pressure heat exchanger is provided having a separated gas inlet in fluid communication with the separated gas outlet and the liquid refrigerant storage means. The natural gas liquefaction means comprises a multi-circuit heat exchanger having a natural gas liquefier circuit and a refrigerant circuit, an expansion valve disposed in the refrigerant circuit and a liquid natural gas tank in fluid communication with the refrigerant circuit.
The method of this invention controls a cycle of the system having a start phase, a run phase and a shutdown phase. In accordance with one embodiment of this invention, a start signal is initiated whereby the refrigerant solenoid valve is operated to provide an initial pressure in the refrigerant circulation means below about 175 psig. Operation of the variable speed motor is initiated whereby operation of the compressor is initiated. The internal solenoid valve is then energized whereby the compressor is operated at full discharge capacity. Operation of the expansion valve is then initiated whereby suction pressure at the suction pressure port is maintained below about 30 psig and discharge pressure at the discharge pressure port is maintained below about 350 psig. Refrigerant vapor from the refrigerant holding tank is then introduced into the refrigerant circulation means until the suction pressure is reduced to below about 15 psig after which flow of the refrigerant vapor from the refrigerant holding tank is terminated. Natural gas is then introduced into the natural gas liquefier, resulting in liquefaction of the natural gas.